When an integrating sphere is used to illuminate a sample in a color or reflectance measuring instrument, the integrating sphere receives light from a light source through an entrance port and the diffusely reflecting interior walls of the integrating sphere reflect the light in multiple reflections so that uniform diffuse illumination is provided over the interior wall surface of the integrating sphere. The integrating sphere is provided with a port designed to receive a sample, the color of which is to be measured. When a sample is positioned over the sample port, the surface of the sample will be illuminated with uniform diffuse illumination multiply reflected from the walls of the integrating sphere. An exit port located on the sphere opposite the sample port will receive diffusely reflected light from the sample and the light passing through the exit port is separated into narrow-wavelength-band components, the intensities of which are measured to determine the reflectance of the sample for each narrow band component. When a sample is placed over the sample port, the color and presence of the sample itself will cause a discoloration and a change in level of the uniform illumination over the interior of the sphere and there is a need to correct the measurements for this level change and discoloration. One way of providing this correction is to provide a second exit port on the sphere positioned to receive light reflected from the wall of the sphere. Since the wall of the sphere is white, the color and intensity of the light reflected from the wall of the sphere when divided into its spectral components and measured will provide data for correcting the color measurement made from the sample.
In U.S. Pat. No. 4,487,504 of Herbert Goldsmith, there is disclosed a system which provides an integrating sphere to illuminate a sample at a sample port and a monochronometer is provided to make measurements of the intensity of the spectral components of the diffusely reflected light. In this instrument, light passing through a sample beam exit port of the integrating sphere is carried by a fiber optic bundle cable to a mechanical fiber optic switching mechanism. A second fiber optic bundle receives light reflected from the wall of the sphere and passing through a reference beam exit port. The second fiber optic bundle at its transmitting end is also connected in the fiber optic beam switching mechanism, which can selectively position either fiber optic bundle cable to transmit through an entrance slit of the monochronometer. When the fiber optic beam switching mechanism is one position, the monochronometer will receive light from the sample and when the switching mechanism is in a second position, the monochronometer will receive light from the wall of the integrating sphere, so that measurements of the spectral components of both the light reflected from the sample and the light reflected from the integrating sphere wall may be made by the monochronometer. As described in the patent, the monochronometer is provided with a pivoted optical grating which disperses the light received through the entrance slit into its spectral components. As the optical grating is pivoted, the spectral components dispersed by the grating are scanned over an exit slit of the monochronometer. A photodetector is positioned to receive the light passing through the exit slit and thus provide measurements of the intensity of the spectral components. The measurements made by the photodetector when the monochronometer is receiving light from the wall of the integrating sphere are used to correct the measurements made by the photodetector when the monochronometer is receiving light from the sample.
The instrument of the above described patent provides an effective system for correcting for the discoloration and level change of the illumination of the interior of the sphere as a result of a sample being positioned over the sample port. However, in order for the measurements to be accurate, the transmitting ends of the fiber optic bundle must be precisely aligned with the entrance slit of the monochronometer each time the beam switching mechanism is operated and must always be aligned to point its transmitting end in precisely the same direction each time a fiber optic bundle aligns with the entrance slit so that the light emitted from the bundle into the monochronometer is always emitted in the same direction from the bundle. As a result, the mechanical fiber optic switching mechanism must be very precisely made and it must maintain its operating precision through the repeated switching actions that occur during use. Wear in the mechanism cannot be permitted to interfere with the operating precision required.
In addition, because the above described instrument of the patent employs a monochronometer with a pivoting grating, in order to measure the entire spectrum, the instrument must take time for the grating to pivot and scan the entire spectrum past the entrance slit. This scanning must be done at least once for the sample, then the beam switching mechanism operated and then done at least once for the reference beam. Each scan by the optical grating takes about seven seconds.